1. Field of the Invention
The present disclosure relates to orthogonal frequency division multiplexing (OFDM), and more particularly, to an apparatus and method for estimating timing offset of OFDM symbols and a method of recovering symbol timing of OFDM symbols.
2. Discussion of the Related Art
Many digital audio broadcasting (DAB) systems, digital multimedia broadcasting (DMB) systems, wireless local area network (WLAN) systems and the like use an orthogonal frequency division multiplexing (OFDM) communication method wherein, data is transmitted using a plurality of orthogonal subcarriers.
FIG. 1 shows a conventional frame structure of OFDM symbols. Referring to FIG. 1, each frame comprises a plurality of OFDM symbols S1, S2, S3, . . . . A NULL symbol is placed at the beginning of each frame. A first OFDM symbol S1 of each frame is a preamble symbol. The preamble symbol includes reference information on the OFDM symbols of each frame. In particular, DAB systems use a phase reference symbol (PRS) as a preamble symbol.
Each symbol interval is divided into a guard interval (GI) and a valid symbol interval (VSI). A copy of a final predetermined portion SF of the VSI is inserted into the GI to prevent inter-symbol interference (ISI).
FIG. 2 is a block diagram of a conventional receiver 200 used by the OFDM communication method. Referring to FIG. 2, a tuner 211 converts a signal received through an antenna (not shown) into an analog intermediate frequency (IF) band signal. An analog-to-digital converter (ADC) 212 converts the analog IF band signal into a digital signal. An interpolator 213 performs interpolation processing on symbol data received from the ADC 212 based on an interpolation frequency clock. A GI remover 214 removes a GI from each OFDM symbol and sends only valid symbols to a fast Fourier transform (FFT) block 215. A differential demodulator 216 performs differential demodulation on symbols which have been FFT-processed by the FFT block 215. A frequency deinterleaver 217 deinterleaves signals that have been output by the differential demodulator 216 and transfers the deinterleaved signals to a subsequent processing block (not shown).
The receiver 200 used by the OFDM communication method performs carrier frequency synchronization and symbol synchronization in order to exactly recover received OFDM symbols.
A carrier frequency error (carrier frequency offset) occurs due to a difference between the frequency of an oscillator used by a transmitter and the frequency of an oscillator used by the receiver 200. Each OFDM symbol comprises a plurality of adjacent subcarriers. Therefore, a small carrier frequency error can greatly influence the recovery of each OFDM symbol. To remove the carrier frequency error and perform the carrier frequency synchronization, the receiver 200 comprises a carrier recovery circuit 220. The carrier recovery circuit 220 comprises a coarse carrier recovery block 221 for compensating for an integral multiple frequency error and a fine carrier recovery block 222 for compensating for a decimal multiple frequency error.
A symbol timing recovery circuit 230 for carrying out the symbol synchronization finds an exact starting point of each OFDM symbol to perform an exact FFT process. The symbol timing recovery circuit 230 comprises a coarse symbol timing recovery block 231 for compensating for an integral multiple timing error and a fine symbol timing recovery block 240 for compensating for a decimal multiple timing error.
The OFDM communication method uses carrier recovery technology for performing the carrier frequency synchronization and symbol timing recovery technology for performing the symbol synchronization.